Electronics are typically cooled from the top of the component with an interface, for example, a Thermal Interface Material (TIM) located between the top of the component that is secured on a Printed Circuit Board (PCB) and a heat sink, such as a conventional heat sink plate. This type of structure is used in many conventional devices, including many telecommunications Outside Plant (OSP) Equipment enclosures. This type of construction has several drawbacks, especially when used with an OSP Digital Subscriber Line Access Multiplexer (DSLAM) enclosure. If the unit requires testing or trouble-shooting, the PCB must be removed from the enclosure forming the heat sink in order to access electronic components. If the heat sink is removed from the component on the PCB for troubleshooting, there is a possibility of damage to the integrated circuit mounted to the PCB from overheating, because of lack of a heat sink. Some components have very high heat fluxes which require that they be heat sunk at all times, even at room temperature.
Also, some surface mount components do not sit flush on the PCB, due to component float during the solder reflow process. There are also thickness tolerances of each integrated circuit package. This requires the use of thick and soft thermal interface pads, especially when using a common heat sink to cool multiple integrated circuits. For example, extremely compliant and thick thermal interface pads are used between the integrated circuits and the heat sink plate to account for the large tolerance stack up that can be generated. This is accomplished to ensure that the components are always in contact with the thermal pad(s), but not overcompressed/damaged by forces imparted by the heat sink plate to the integrated circuit(s), or to other components that may be located within the thermal pad area. Also, the cost of the thermal interface material is a function of both thermal conductivity and hardness, as well as, sheer volume of the material used. The higher thermal conductivity material is typically more expensive than lower conductivity material. The softer material is typically more expensive than the harder material. Additionally, it is difficult to generate accurate thermal images of components when the heat sink is mounted to the component side of the PCB, and thus, positioned over the component.
There is also an issue with such construction because it is difficult to interconnect PCB's together. The heat sink covers the components on the component side, making interconnection difficult. Thus, an issue arises of how to connect multiple PCB's together in an outside plant equipment enclosure.